If there is still any confusion about chart datums. Imagine that the chart is just an illustration, with no latitude or longitude lines on it. No numbers, no grid. Now add a wire grid, of the lat/long lines, laying on top of the chart and able to be moved anywhere.
The question is, where do you move it? Which corner becomes "zero, zero" or "80, 30" or whatever your position is?
The datum tells you which global positioning (not to be confused with GPS) scheme was used to prepare the map and where the author of the map thinks the numbers should be aligned. Because it is damned hard to physically measure positions circling around the world, and the world is not a perfect sphere, there have been different constructs for positions over the years.
So on a paper chart, there's a note (usually!) that says which datum was used to make that chart. There are formulas that can convert from one datum to another, based on the known differences in offset in different locations. Of course, that's fairly cumbersome to apply to a paper chart.
But with electronic charts
, the chart viewing software
and the positioning software
(i.e. GPS) allow you to select the datum that they are using, with the rash assumption that it is usually WGS87 for GPS
users. It won't always be, and if someone makes the mistake of "getting" charts
from an unknown source, or forgetting to change a datum setting, they can wind
Heck, even if you know the datum you can be helixed because some software uses the DD.MM.SS format while others use DD.MM.mm or DDdddd and there isn't always any prominent note about it. (Which is why properly noting these things matters.)
Then there are charts that are simply incorrect, based on old inaccurate surveys. They are often known well locally, but unknown to travelers. There's a USGS topo map that was reproduced for years by a major hiking club with "NOTE TO ENEMY BOMBERS" printed at the bottom, mentioning a displacement
of several miles. And I've seen a NOAA chart show structures on land 1/3 mile off from their GPS
position, supposedly on the same datum in a well-charted area.
Charts? Yeah, well, using a chart instead of a gas station map doesn't buy you anything unless you are aware that they both have certain limitations. In the States we're probably spoiled by GPS and so many charts being drawn to the same WGS87 datum.
But you only need to drive around with a Garmin
(or any other brand) for a month to find out that even when everything is on the same standard, some things just aren't where the box says they are.
Or as the Mad Hatter said, "But it was the very best of butter!"